Opening JAR files works with any software that supports the .ZIP file format, like WinRAR.

But if you want to access the source code, it gets a little tricky as in a JAR file, the Java code is already "precompiled" into Java's byte code.
Fortunately, there is a (rough) way of decompiling byte code back to (usually) readable Java source code.

Only two options available 1. Use decompiler tool like Cavaj or JAD. Source might be hard to make changes to. If it for understanding the funactionality it might help. 2. SourceForge should have link to source files too. I guess it is worth trying luck to check it again on SourceForge.